Phoronix has a new article with some stats on Spectre/Meltdown mitigation performance impact on Linux 5.0, using their test suite:
[…]Of 57 benchmarks tested on these three systems with the Linux 5.0 kernel, the Core i9 7980XE performance was down by about 13% based upon the geometric mean of all the test results. The Intel Core i7 8086K performance was down by 17% with these out-of-the-box protections for Spectre and Meltdown. The AMD Ryzen 7 2700X performance with its default Spectre mitigations was lower by just 3%. Should you choose to go against the security assessment and wish to recover from these performance losses, reverting the mitigations as tested can easily be done by some boot parameters albeit no single switch. Now with Microsoft shipping Retpolines for revising their Spectre V2 mitigation, some additional Spectre/Meltdown tests will be coming up soon on Phoronix.